In a measurement of an absorbance of a liquid sample or gas sample, an optical analyzer is normally used, such as an ultraviolet-visible spectrophotometer or photodiode array detector. For example, an ultraviolet-visible spectrophotometer commonly uses a deuterium discharge tube as the light source for the ultraviolet region and a halogen lamp, as the light source for the visible region. In recent years, an ultraviolet-visible spectrophotometer using a xenon flash lamp (which has a longer life than the halogen lamp or deuterium discharge tube) has also been developed. In any case, optical analyzers using those light sources are normally configured so that monochromatic light is extracted by a monochromator using a diffraction grating or similar device and cast into or onto a sample, or so that light obtained from a sample is introduced into a light-dispersing device and dispersed into wavelengths components, which are then partially or entirely introduced into and detected by a detector.
In recent years, with the advancement and rapid spread of the light-emitting diode (LED) technology, LEDs have also been increasingly used as light sources in optical analyzers. Since LEDs have a comparatively narrow peak in their emission spectra, they are less suitable for applications which require the scan of a wide range of wavelengths. However, LEDs are suited to optical analyzers which casts a specific wavelength of light into or onto a sample, as in the case of an absorptiometer or fluorometer. LEDs are not only far more inexpensive than the previously mentioned light sources, but also have a long life and operate with high reliability. On the other hand, in general, the amount of light emitted from an LED considerably changes with the ambient temperature. In optical analyzers, such a change in the amount of light makes the measured result less accurate. Therefore, attempts have been made to reduce such an influence of the temperature dependency of the amount of light by controlling the temperature of the LED or controlling the drive current to the LED according to the temperature change so as to maintain the amount of light at a fixed level.
Nevertheless, those methods are not sufficient for performing a measurement with high accuracy. Accordingly, in an analyzer described in Patent Literature 1, in addition to the LED temperature control, a configuration is adopted in light of the fact that the degree of the temperature-dependent change in the amount of LED light is more noticeable at shorter wavelengths than at longer wavelengths. According to the configuration, an optical filter is placed in an optical path of the light emitted from the LED, for blocking light within a wavelength region shorter than the peak wavelength in the emission spectrum of the LED, i.e. within a region where the amount of light is particularly temperature dependent.
By the previously described methods, the problem of the measurement accuracy due to the temperature dependency of the amount of light emitted from an LED can be solved to a certain extent. However, even if such a factor is removed, an optical analyzer using an LED as a light source is inferior to optical analyzers using conventional light sources in that the detection signal contains a greater amount of noise and drift. Therefore, to further improve the measurement accuracy, it is essential to reduce the noise and drift originating from the light source.
As stated earlier, the peak width of an LED emission spectrum is normally narrow. This has led to the expectation that it may be possible to directly use LED light as measurement light without changing it to monochromatic light using an expensive light-dispersing device. However, for example, if light emitted from an LED is directly used as measurement light for absorbance analysis, the linearity of absorption will be low, in particular, within a high-absorbance region.